Interpretive Summary: Application of nitrogen (N) is essential for sustaining food, feed, and fiber production, but in the Midwestern U.S., non-point-source pollution of surface water and groundwater resources has been identified as an unintended off-site impact of this agricultural management practice. Low crop recovery of applied N is a major reason for not only non-point- source pollution, but also economic loss to farmers throughout the region. In this context, field studies evaluating the impact of agricultural management practices on soil and water quality are needed to provide the information for developing more economically and environmentally sustainable soil and crop management systems. Tillage is a soil management factor that strongly affects N use efficiency through its impact on water entry, retention, and subsequent leaching of N through the soil profile. Many studies have shown environmental benefits and greater corn yields with late-spring N applications based on soil tests for N carried out in the late spring. This study was conducted to evaluate the combined effects of tillage and N management on residual soil nitrate N and crop yield. We found that applying N in the late spring based on soil tests increased corn yield compared with a single, pre-plant N application regardless of the tillage practice. We also found that more intensive tillage increased corn yield compared with no-till regardless of the N management practice. This research provides new information about the impact of N management and tillage on corn yield which will ultimately be used to optimize agricultural management practices for corn-soybean production systems in the Midwest.

Technical Abstract:
Tillage and N-management impact crop yield and off-site transport of NO3-N. This 6-year field study on tile-drained Clyde-Kenyon-Floyd soils in northeast Iowa was conducted to quantify corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) yield and residual soil N03-N. Eight treatments (chisel plow vs. no-tillage by pre-plant vs. late-spring N-management for both corn and soybean phases of the rotation) were evaluated using a randomized complete block design. Pre-plant N was applied by injecting liquid urea-ammonium nitrate solution (UAN) at a rate of 110 kg N ha**-1. Late-spring soil test based N-rates averaged 179 and 156 kg N ha**-1 for no-till and chisel plow, respectively. Average corn yield on chisel plots was higher than with no-tillage for both pre-plant (7.9 vs. 6.9 Mg ha**-1) and late-spring (8.6 vs. 8.1 Mg ha**-1) N-management. Average soybean yield where corn had received pre-plant N (3.6 Mg ha**-1) was greater than where late-spring N-management (3.4 Mg ha**-1) was used. Residual tillage effects did not affect soybean yield. The average residual soil NO3-N to a depth of 1.2 m following corn was 31 and 24 kg N ha**-1 for late-spring and pre-plant N-management treatments. Residual soil NO3-N following soybean was lower in no-till (28 kg N ha**-1) than chisel (37 kg N ha**-1) plots. Average over-winter changes in residual soil NO3-N were greatest in corn plots previously fertilized with a single pre-plant application (+13 to 18 kg N ha**-1) and most variable following soybean in plots where corn was fertilized based on late-spring nitrate test values (-8.5 to +6.3 kg N ha**-1). A more complete understanding of N cycling during the winter months will help define efficient N-management strategies.